The invention relates to the field of regenerating spent pickle liquors, and more particularly to the regeneration of spent hydrochloric acid pickle liquors that have been used for ferrous metal pickling by recovering ferrous chloride from the spent pickle liquors at very low temperatures.
Pickling of ferrous metals, such as steel strip, sheet, tubes or wire, to remove metal oxides and scale has typically been performed with a commercial hydrochloric acid pickle liquor that has been heated to about +200 degrees Fahrenheit by closed circuit steam-acid heat exchangers. A ferrous chloride solution is a principal by product of the pickling process.
High temperature systems for the closed-loop regeneration of spent hydrochloric acid pickle liquors that have been used to pickle ferrous metals are known. A roasting process has been conventionally employed in which the ferrous chloride within the spent pickle liquor is hydrolyzed in a reaction that takes place at approximately +840 degrees Fahrenheit to produce iron oxide and hydrogen chloride gas. The hydrogen chloride gas is then absorbed into water to form a suitable concentration of aqueous hydrochloric acid that can be returned to the pickling line as a regenerated pickling liquor. The high temperatures involved in such roasting processes are disadvantageous for a number of reasons, including the relatively high maintenance and operating costs attendant high temperatures, and the fact that at such high temperatures organic acid inhibitors typically utilized in acid pickle liquors are largely destroyed. Another known alternative has been a distillation process, but the same disadvantages of high distillation temperatures, the attendant high maintenance and operating costs, and the destruction of organic acid inhibitors, are present.